PLEISTOCENE STRATIGRAPHY IN EAST-CENTRAL IOWA

G.R. Hallberg


Iowa Department of Natural Resources, Geological Survey Bureau,
Technical Information Series 10, 1980, 168 p.

ABSTRACT


The Pleistocene stratigraphy in east-central Iowa was investigated from exposures and from drill-cores which penetrated the entire Pleistocene sequence, from the land-surface to the bedrock. From this analysis a new, multiple classification stratigraphic framework was developed for the tills and related deposits which occur stratigraphically below the sediment of the Yarmouth-Sangamon and Late-Sangamon Paleosols.

Classically, these sediments have been referred to as Kansan and Nebraskan in both a time and rock-stratigraphic sense. In this report they are redefined, and are referred to undifferentiated Pre-Illinoian Stages for their time-stratigraphic classification. Rock-stratigraphically these deposits are subdivided into the Alburnett and Wolf Creek Formations, based on distinct differences in clay mineralogy. Both formations are composed principally of multiple basal tills and intertill stratified sediments. Multiple tills occur within the older Alburnett Formation. These till units are not formally subdivided into members because no properties have yet been recognized which would allow correlation. The Wolf Creek formation is subdivided into the Winthrop, Aurora, and Hickory Hills Till Members on the basis of physical stratigraphy, differences in texture, sand-fraction lithologies, and matrix carbonates.

Two soil stratigraphic units are also named. The Westburg Paleosol occurs below the Winthrop Till Member of the Wolf Creek Formation and is developed in deposits of the Alburnett Formation or older rock units. The Dysart Paleosol occurs below the Hickory Hill Till Member of the Wolf Creek Formation. Other weakly developed soils occur between other till units, but they are not formally classified at this time because their significance is unclear.

Substantial erosion has occurred in this region of old glacial deposits. The area is marked by the widespread development of the Iowan Erosion Surface of Wisconsinan-age. Significant pre-Wisconsinan erosion also occurred. The Late-Sangamon surface and paleosol are developed on everything from the Hickory Hills Tills Member to the much older tills of the Alburnett Formation.

The relationship between the landscape and stratigraphy is analogous to an area of dissected, relatively flat-lying sedimentary rocks. As the landscape drops in elevation from the Wapsipinicon-Maquoketa River and the Iowa-Cedar River divides to the lower elevations of the Cedar River basin, the younger Wolf Creek Formation deposits are truncated by erosion and only locally preserved. In this lower elevation area the stratigraphically lower Alburnett Formation is widely exhumed at the land surface. In these Pleistocene deposits there are no "cliff-forming" units to provide escarpments and make this relationship obvious.

At the local level, drainage basins are marked by multi-leveled, stepped erosion surfaces, which also cut across the stratigraphic units. These relationships between the landscape and the stratigraphy, on both the local and regional level, must be recognized when working in older, eroded glacial terrane. It cannot be assumed, as most historical work in this area has done, that the uppermost till unit is the same across the area. In different landscape positions, different portions of the stratigraphic section are preserved and exposed.